Add policy check if cross reference system notes are accessible

See merge request gitlab/gitlabhq!3426

Cancel all running CI jobs when user is blocked

See merge request gitlab/gitlabhq!3436

Filter not accessible label events

See merge request gitlab/gitlabhq!3440

Label events may use cross-project or cross-group references,
if the projects are not accessible by user, we don't show these
label events.

[ci skip]

This prevents a MITM attack where attacker could
still access Git repository if any jobs were
running long enough.

[ci skip]

[ci skip]

[ci skip]